Problem Description
bobo has a permutation p1,p2,…,pn of 1,2,…,n.
Knowing m extra constraints of form pai<pbi, bobo wanna count the number of different permutations modulo (109+7).
It is guaranteed that there is at least one such permutation.
Input
The input consists of several tests. For each tests:
The first line contains n,m (1≤n≤40,0≤m≤20). Each of the following m lines contain 2 integers ai,bi(1≤ai,bi≤n).
Output
For each tests:
A single number denotes the number of permutations.
Sample Input
3 1
1 2
3 2
1 2
2 3
Sample Output
3
1
